EMBRYO

After a blastocyst has found a comfy home in the uterus, it begins to develop into an embryo, which is attached to the placenta and floats inside fluid-filled membranes called the amniotic sac. During this stage, the cells of an embryo begin to perform different functions and the heart, internal organs, arms, legs and lungs take shape. All organs except for the brain and spinal cord are completely formed by 12 weeks into the pregnancy. For this reason, the first 12 weeks stage are when the baby is most vulnerable to birth defects or the ill effects of drugs, viruses or radiation.